A screw machine such as an extruder employed for producing plastics materials comprises an elongated sleeve surrounding one or more screws. If the machine has a single screw, the latter is placed in a cylindrical bore of the sleeve. In the case of machines having two screws, the sleeve is provided with two cylindrical bores having parallel axes, the distance between the axes being less than the diameter of the bores so as to form two intersecting lobes in which the two screws are disposed, these screws engaging into one another. If the screws are driven in the same direction, they are identical. On the other hand, they are symmetrical if they are driven in opposite directions. For this purpose, the screws are driven by a mechanism placed at one end of the sleeve.
As it may be of interest to vary the screw threads of the screws, the latter are sometimes formed by screw-threaded elements in the form of hollow collars mounted on a central driving shaft so as to form a stack of adjoining elements which are connected to the shaft to be driven in rotation by the latter, for example by means of splines projecting from the shaft and engaging in corresponding grooves formed in the inner wall of the collars. The latter are stacked and clamped between two abutments placed at the two ends of the shaft, namely usually a fixed abutment placed at one end and a clamping nut screwed on a screw thread formed on the other end of the shaft. With this arrangement, one or more screw-threaded elements may be easily changed for modifying the pitch of the threads or for replacing a worn element.
The material driven in the screws is usually put under pressure and therefore has a tendency to spread apart adjacent elements and to infiltrate therebetween. It may then reach the splines and possibly harden. The elements then become difficult to demount and the splined shaft must be cleaned each time the elements are changed and this operation may be rather long. It has been attempted to overcome this drawback by placing sealing gaskets between the screw-threaded elements, but experience has shown that, when the pressures are high, this drawback is difficult to avoid. Indeed, the clamping nut is used solely for interconnecting the elements and, as it must be easily demounted, it cannot exert sufficient pressure to oppose in a definite manner a separation of the elements which would allow the infiltration of material.
Further, the shafts of the screws must of course be mounted in bearings. When the machine is an extruder provided with dies at its downstream end, the screws are held by bearings solely located on the driving end. At their other end, they are held centered relative to the sleeve by the pressure of the extruded material. In other applications, the machine does not operate as an extruder but as, for example, a mixer-kneader. The material may not be put under pressure at the downstream end and may be discharged through a lateral orifice in the sleeve. In this case, it is more normal to carry each screw by means of two bearings placed at both its ends. However, in certain processing methods, it is conventional to pass the material through pressurizing zones arranged along the shaft and for example constituted by retarding zones comprising closer pitch or reversed threads. In this case, the pressure of the material in the zone in which the screws are engaged in one another produces a force which tends to move these screws apart so that they are subjected to bending stresses. Consequently, there is produced in the central part a deflection which, it is often considered, may reach three times the radial clearance of the screws. Therefore, the screw-threaded elements are subjected in the engaging section to high axial compression stresses and, on the other hand, tend to move away from each other on the periphery. This ofsetting of the axial compression force may result in a bruising effect on the bearing surfaces of the elements. Moreover, owing to this slight bending of the screws, the screw threads are not evenly worn.